As is well known, wireless communications channels are subject to time-varying multipath fading, and it is relatively difficult to increase the quality, or decrease the effective error rate, of a multipath fading channel. While various techniques are known for mitigating the effects of multipath fading, several of these (e.g. increasing transmitter power or bandwidth) tend to be inconsistent with other requirements of a wireless communications system. One technique which has been found to be advantageous is antenna diversity, using two or more antennas (or signal polarizations) at a transmitter and/or at a receiver of the system.
In a cellular wireless communications system, each base station typically serves many remote (fixed or mobile) units and its characteristics (e.g. size and location) are more conducive to antenna diversity, so that it is desirable to implement antenna diversity at least at a base station, with or without antenna diversity at remote units. At least for communications from the base station in this case, this results in transmit diversity, i.e. a signal is transmitted from two or more transmit antennas.
S. M. Alamouti, “A Simple Transmit Diversity Technique for Wireless Communications”, IEEE Journal on Selected Areas in Communications, Vol. 16, No. 8, pages 1451-1458, October 1998 describes a simple transmit diversity scheme using space-time block coding (STBC). For the case of two transmit antennas, complex symbols s0 and −s1* are successively transmitted from one antenna and simultaneously complex symbols s1 and s0* are successively transmitted from the other antenna, where * represents the complex conjugate. These transmitted symbols constitute what is referred to as a space-time block.
It is also known to use various coding schemes in order to enhance communications. Among such schemes, it has been recognised that so-called turbo coding (parallel concatenated convolutional coding) enables iterative decoding methods to achieve results which are close to the Shannon limit for AWGN (additive white Gaussian noise) communication channels. A turbo coder uses two, typically identical, recursive systematic convolutional (RSC) component coders, signals to be transmitted being supplied directly to one of the component coders and via an interleaver to the other of the component coders. Accordingly, it would be desirable to combine turbo and space-time coding techniques in the same transmitter.
V. Tarokh et al., “Space-Time Codes for High Data Rate Wireless Communication: Performance Criterion and Code Construction”, IEEE Transactions on Information Theory, Vol. 44, No. 2, pages 744-765, March 1998 describes various convolutional, or trellis, codes which can be used with two or more transmit antennas to provide the advantages of trellis (convolutional) coding and space-time coding. Although these codes are considered optimal for maximum diversity gain, they are not necessarily optimal for coding gain. Furthermore, these codes are non-recursive. In contrast, it is well established that the best efficiency for turbo coding is achieved using recursive codes. Consequently, the codes described by Tarokh et al. are not suitable for use in a turbo coding arrangement.
P. Robertson et al., “Bandwidth-Efficient Turbo Trellis-Coded Modulation Using Punctured Component Codes”, IEEE Journal on Selected Areas in Communications, Vol. 16, No. 2, pages 206-218, February 1998 describes a turbo coder using Ungerboeck and multidimensional TCM component codes, in which the interleaver operates on groups each of m information bits. For each step corresponding to a group of m information bits, a signal mapper associated with each component coder produces n symbols, where n=D/2 and D is the signal set dimensionality; for example D=2 or 4 and n=1 or 2. An n-symbol de-interleaver de-interleaves output symbols from the second component coder, and a selector alternately for successive steps selects symbols output from the first component coder and symbols from the de-interleaver and supplies them to a single output path. This arrangement does not provide transmit diversity and this document is not concerned with space-time coding.
G. Bauch, “Concatenation of Space-Time Block Codes and “Turbo”-TCM”, Proceedings of the International Conference on Communications, ICC'99, pages 1202-1206, June 1999 describes two types of turbo trellis coded modulation (TCM) coder, whose output is supplied to a space-time block coder, so that the turbo-TCM and STBC arrangements are simply concatenated with one another. One of these two types of turbo TCM coder is as described by Robertson et al. (to which reference is made for details) as discussed above using Ungerboeck codes and providing one symbol at the output of the mapping function, but the Bauch illustration of this does not show the symbol de-interleaver. This Bauch publication does not discuss multidimensional component codes.
A continuing need exists to provide further improvements in wireless communications.